1. Field of the Invention
The present invention relates to a piston having a discontinuous seal ring and used in a compressor, and in particular, relates to an improvement of a sealing function at an initial stage of a compression stroke in a compressor using reciprocable pistons.
2. Description of the Related Art
In a compressor using reciprocable pistons, such as a crank shaft type compressor, a swash plate type compressor, and an oscillating plate type compressor used in particular in an air conditioning system of an automobile, many proposals have been made with regard to reducing the size of the compressor. To accomplish this, the cylinder block and piston are made from an aluminum alloy, and in the design of such an aluminum alloy compressor a relatively large clearance between the piston and the cylinder bore has been adopted, to avoid scouring due to a sliding engagement between the aluminum alloy parts, and in this case, a piston ring made of a plastic material is often used to seal a gap between intersliding parts. Such a piston ring usually has a rectangular cross-section conforming to the shape of a ring groove on the outer surface of the piston, and is a discontinuous ring having opposite ends which meet when the piston ring is circumferentially compressed.
An example of a conventional piston is shown in FIG. 7 of the attached drawings, in which a piston 500 is fitted in a cylinder bore 502 to form a compression chamber 508 on at least one side of the piston 500, and a piston ring 504 is inserted in a ring groove 506 with side surfaces 506c and a bottom surface 506d, so that clearances are provided between one of the side surfaces 506c of the ring groove 506 and the end surface of the piston ring 504, and between the bottom surface 506d of the ring groove 506 and the inner surface of the piston ring 504, when the piston 500 moves in the direction of the arrow F during the compression stroke of the compressor. The pressure generated in the compression chamber 508 is introduced to these clearances in the ring groove 506 and thereby applies a back pressure to the piston ring 500, as shown by the arrow G, to cause the discontinuous piston ring 504 to expand and come into closer contact with the wall of the cylinder bore 502.
The initial shape of such a discontinuous piston ring 504 is usually slightly larger than the internal shape of the cylinder bore 502 and is inserted in the cylinder bore 502 under an initial tension, and thus the piston ring 504 should be in close contact with the inner wall of the cylinder bore 502. The piston ring 504 made of a plastic material, however, is not as stiff as a piston ring made of a metal, as often used in an internal combustion engine, and the initial tension of the plastic piston ring 504 is weak. Accordingly, the design is such that the pressure G from the compression chamber 508 presses the piston ring 500 into closer contact with the wall of the cylinder bore 502, as described above, and the action of the pressure G of pressing the piston ring 500 into closer contact the wall of the cylinder bore 502 is important to the establishing of a good sealing by the plastic piston ring 504. Nevertheless, the pressure in the compression chamber varies in accordance with the position of the piston 500, and in particular, the pressure is weak at an initial stage of the compression stroke of the piston 500, and thus the pressure G does not act to sufficiently expand the plastic piston ring 504 until the pressure G is increased to a much higher level. Accordingly, a problem of the sealing capability arises at the initial stage of the compression stroke in a compressor, and of a loss of compression stemming therefrom.